As shown in FIG. 1, in a typical PWM power converter 100, an error amplifier 102 compares a feedback signal Vfb derived from an output voltage Vout with a reference signal Vref to produce an error signal EO, a PWM modulator 106 compares the error signal EO with a constant slope linear oscillating ramp signal RAMP provided by a ramp generator 104 to produce a PWM signal, the PWM signal switches two transistors M1 and M2 with a gate driver 108, and the duty of the PWM signal determines the ON time and OFF time of the transistors M1 and M2, so as to regulate the output voltage Vout. The ratio of the supply voltage Vin to the amplitude of the ramp signal RAMP, i.e. Vin/Vramp, is called modulation gain of the PWM loop, and is closely related to the response speed and the stability of the PWM loop. Besides, the loop gain of the power converter 100 is proportional to the product of the feedback factor of a compensation circuit composed of resistors R1, R2, R3 and R4 and capacitors C1, C2 and C3, the gain of the error amplifier 102, and the modulation gain, i.e. (Vref/Vout)×(Vin/Vramp), so the modulation gain also influences the transient response of the power converter 100.
FIG. 2 shows various signals in the circuit of FIG. 1. At time t1, the feedback signal Vfb drops down due to some reasons such as a sudden decrease of the input voltage Vin and a decrease of the output voltage Vout caused by a rapid current draining by the load, and causes the difference between the feedback signal Vfb and the reference voltage Vref lager, so the error signal EO rises up obviously after time t1. At time t2, the error signal EO keeps increasing and becomes higher than the ramp signal RAMP, so the PWM signal changes from low to high. When the PWM signal is high, the transistor M1 is on while the transistor M2 is off, so the output Vout is charged by the power source Vin. At time t3, the feedback signal Vfb rises up again toward the original level with the increasing output voltage Vout. This waveform diagram substantially shows the process of the transient response of the power converter 100.
Several arts have been proposed to improve the stability of the modulation gain. U.S. Pat. No. 6,522,115 to Greitschus provides a non-linear ramp signal to replace the linear ramp signal for the comparator; however, it requires one more pin for the controller chip to monitor the inductor current, and the PWM loop is more difficult to design and modify. U.S. Pat. No. 6,593,725 to Gallagher et al. proposes a pulse generator to control the ramp signal and the PWM signal, but it needs to monitor the supply voltage and has complex circuit. U.S. Pat. No. 6,049,473 to Jang et al. not only controls a non-linear ramp generator by the PWM signal, but also monitors the supply voltage.
Therefore, it is desired a method and apparatus without monitoring the supply voltage in order to improve the stability and response speed of the PWM loop.